Wellbore drilling through earth formations for extracting fluids includes making a well plan or prognosis prior to starting drilling. A well plan generally includes the spatial position of earth formations that the wellbore is to penetrate, the path or trajectory of the wellbore from the earth's surface to the prospectively penetrated formations, and the depths in the wellbore, and sizes thereof, of protective pipe, or casing, which is used to protect the penetrated formations and to provide a conduit for formation fluids to flow to the earth's surface.
The ultimately planned wellbore trajectory depends on, among other factors, available drilling locations at the earth's surface (or the geographic location of a drilling vessel or drilling platform in offshore wells), and the relative spatial positions of the prospectively drilled formations. The casing depths depend on, among other factors, fluid pressures in the pore spaces of all the permeable formations to be drilled along the trajectory, formation fracture pressures and mechanical properties, and the ultimate depth and lateral extent of the wellbore.
In developing a well plan, a wellbore designer takes into account the possibility that the formations to be penetrated have excessive fluid pressure in the pore spaces, and whether exposed (already drilled), but shallower depth formations, are able to withstand (avoid fracturing by) hydrostatic pressure needed in the wellbore to control such excess pressures. Failure to withstand the hydrostatic pressure in the wellbore may result in loss of drilling fluid (“lost circulation”). Similarly, the well designer must consider whether exposed formations have fluid pressure such that control of higher pressure formations in the wellbore may increase the risk of having a drilling assembly become stuck in the wellbore due to differential fluid pressure across the lower pressure formations (“stuck pipe”). Other causes of stuck pipe can include caving of susceptible formations, which may result from chemical interaction of the formation with components of the drilling fluid, or from penetrating a formation having high mechanical stresses therein.
Failure to have sufficient hydrostatic pressure in the wellbore when drilling through certain formations may result in fluid influx to the wellbore (taking a “kick”). Taking a kick can be dangerous, particularly when the kick includes large quantities of gas, because hydrostatic pressure can be further reduced in the wellbore, causing consequent increase in influx. The ultimate result may be a “blowout” or uncontrolled discharge of fluid from the wellbore, or the kick may ultimately fracture a shallower, weaker formation.
Mechanical properties of the formations to be drilled, in combination with the well trajectory, can affect the dynamic response of the drilling assembly. In some cases, drill bits can wear out prematurely, or excessive vibrations in the drilling assembly can result in catastrophic component failure during wellbore drilling.
Undesirable occurrences during drilling, such as the foregoing examples, may be broadly characterized as “drilling hazards”. In developing a well plan according to prior art methods, the wellbore designer uses available information about the subsurface earth formations and the proposed trajectory to avoid hazards which are known to exist or which have a very high probability of being encountered during drilling. For example, drilling through certain formations with insufficient hydrostatic pressure in the wellbore will most likely result in taking a kick in those formations. Prior art wellbore design techniques to avoid kicks include having sufficient hydrostatic pressure in the wellbore when drilling through such formations, while making sure that the hydrostatic pressure does not exceed estimated fracture pressure of any exposed earth formations. Information used in wellbore design to avoid known hazards includes earth formation characterization information, such as well logs and cuttings analysis from other wellbores in the vicinity of the proposed wellbore, surface seismic surveys, and formation pressure and/or production tests from nearby wellbores. For proposed wellbores for which such data are unavailable, the wellbore designer may use surface seismic survey information to estimate formation fluid pressures and depths at which such formations may be penetrated. Data from wellbores drilled in more distant areas may also be used where formation mechanical properties are to be estimated for the prospective wellbore.
A limitation of prior art wellbore design techniques is that drilling hazards are generally characterized as either existing or not with respect to a particular proposed wellbore design. A proposed wellbore design which is determined to almost certainly have such a drilling hazard may be adjusted by the wellbore designer to avoid the hazard. Similarly, wellbores being actively drilled may give indication of the near certainty of encountering a drilling hazard, which may result in a while-drilling modification to the well plan. Data acquired during drilling, such as from measurement-while-drilling and logging-while-drilling instruments may be used to adjust the well plan when an otherwise unforeseen drilling hazard is identified to high probability of occurrence. Actually encountering a drilling hazard typically results in at least some additional time and expense rectifying the damage from encountering the hazard.
In each of these situations, the wellbore designer does not have the ability, using prior art techniques, to determine the probability of encountering a drilling hazard for any particular well plan, to determine the likely severity of the consequences if such a hazard is encountered, and the magnitude of the economic cost or work cost necessary to repair any damage caused by encountering the drilling hazard.
Prior art drilling performance evaluation systems generally include methods for simulating the penetration of a bit through earth formations having selected mechanical properties, where selected drilling assembly and drilling operating parameters are entered into the system. The prior art systems provide a well designer with some ability to optimize the design of the drilling assembly and the operating parameters for particular types of earth formations. Typical prior art drilling evaluation and analysis systems are described, for example, in U.S. Pat. No. 6,021,377 issued to Dubinsky et al; U.S. Pat. No. 6,109,368 issued to Goldman et al; U.S. Pat. No. 5,704,436 issued to Smith et al; U.S. Pat. No. 5,794,720 issued to Smith et al; U.S. Pat. No. 5,318,136 issued to Rowsell et al; U.S. Pat. No. 6,002,985 issued to Stephenson; U.S. Pat. No. 5,730,234 issued to Putot; and U.S. Pat. No. 5,812,068 issued to Wisler et al. See also, D. Dashevskly et al, Application of Neural Networks for Predictive Control in Drilling Dynamics, paper no. 56442, Society of Petroleum Engineers, Richardson, Tex. (1999). None of the prior art describes any method or system for characterizing the risk and consequences of encountering drilling hazards.
Another technique for characterizing drilling hazards known in the art is called “3-D visualization”. Generally speaking, a number of different types of geophysical interpretation, such as seismic, well log analysis, cuttings analysis and prior well histories are included in a common model of earth formations in the area surrounding a well to be drilled. The common earth model can be displayed in any one of a number of three dimensional computer generated graphic forms. Various proposed wellbore trajectories may be inserted onto the computer graphic representation. A description of 3-D visualization as is relates to wellbore planning can be found, for example, in, J. Holt et al, Mungo Field: Improved Communication through 3D Visualization of Drilling Problems, paper no. 62523, Society of Petroleum Engineers, Richardson, Tex. (2000). 3-D visualization techniques known in the art do not have the capability of predicting drilling hazards, so their usefulness is generally limited to the visualization itself.
What is needed is a system which enables a well designer to determine, from any one or more of a plurality of data sources, potential drilling hazards in a prospective wellbore, and for each of these hazards, a determination of the likelihood that each such hazard will be encountered, and a likely magnitude of consequences of encountering the hazard or the severity of the hazard. It is also desirable to have a system which may include data obtained during drilling of a wellbore which enables redetermination of the foregoing aspects of possible drilling hazards, and enables a wellbore designer to enter changes to a well plan therein to evaluate the likely effect of these changes in the well plan. Finally, it is desirable to have a system which enables a wellbore designer to optimize a well plan with respect to most likely consequences of encountering certain drilling hazards.